Carl Fredrik Forsberg

The Storegga Slide Complex; Repeated Large Scale Sliding in Response to Climatic Cyclicity

The Holocene Storegga Slide is the last of a series of slides occurring in the same area during the last 500ky. The objectives of the present paper are to present the current understanding of the trigger mechanisms and development of the Storegga Slide, and to show the link between the sliding and Pleistocene climatic fluctuations in the area. Instability is created by the rapid loading of fine-grained hemipelagic deposits and oozes by rapid glacial deposition during peak glaciations. Postglacial earthquake activity was the most likely trigger. Although slide development is complicated and involves a number of slide mechanisms and processes, the overall development is retrogressive, starting at the mid- to lower slope. Sliding stops when the headwall reaches the flat lying, overconsolidated glacial deposits of the shelf.

Late glacial palaeoceanography of Hinlopen Strait, northern Svalbard

Timing and structure of the Late and post-glacial development of the northern Svalbard margin, together with the initial influx of Atlantic water into the Arctic Ocean are still very poorly constrained. We investigated a sediment core (NP94-51) from a high accumulation area on the continental shelf north of Hinlopen Strait with the purpose of resolving the timing and structure of the last deglaciation. Detailed analyses of ice-rafted detritus, benthic and planktonic foraminiferal fauna, diatom flora, grain size and radiocarbon dates are used to reconstruct the palaeoceanographic evolution of the area. Our results indicate that the disintegration of Hinlopen Strait ice and possibly the northern margin of the Svalbard Ice Sheet commenced between 13.7 and 13.9 14C Ky BP. Influx of subsurface Atlantic waters into the area (12.6 14C Ky BP) and the retreat of the sea ice cover, with the accompanying opening of the surface waters (10.8 14C Ky BP), happened at different times and both much later than the disintegration of the ice sheets. The transition into the Holocene shows a two-step warming.

Identification of Weak Layers and Their Role for the Stability of Slopes at Finneidfjord, Northern Norway

The 1996 Finneidfjord landslide, which took four human lives in northern Norway, initiated along a weak layer in the fjord-marine sediments before developing retrogressively across the shoreline. The integration of results from sediment cores, free-fall cone penetrometer tests and high-resolution 3D seismic data indicates that the slide-prone layer is a regional bed likely sourced from clay-slide activity in the catchment of the fjord. The sediments in this regional layer are softer and more sensitive than the typical bioturbated, fjord-marine deposits, which explains their role in slope instability. In addition, biogenic gas in the stratified event bed may further affect its geotechnical properties. Similar, fine-grained, stratified beds with comparable origin and properties occur in other Norwegian fjords. They are presumably also present along coastlines of other previously glaciated margins, where they could contribute to mass movements.

Evidence for orbitally controlled size variations of the East Antarctic Ice Sheet during the late Miocene

Ocean Drilling Program Site 1165 penetrated drift sediments on the East Antarctic continental rise and recovered sediments from a low-energy depositional environment. The sediments are characterized by prominent alternations between a green to greenish-gray diatom-bearing hemipelagic facies and gray to dark gray hemiturbiditic facies. Our investigation of an upper Miocene section, using high-resolution color spectra, multisensor core logs, and X-ray fluorescence scans, reveals that sedimentation changes occur at Milankovitch orbital frequencies of obliquity and precession. We use this finding to derive an astronomical calibrated time scale and to calculate iron mass-accumulation rates, as a proxy for sediment-accumulation rates. Terrigenous iron fluxes change by as much as 100% during each obliquity cycle. This change and an episodic pattern of enhanced ice-rafted debris deposition during times of deglaciation provide evidence for a dynamic and likely wet-based late Miocene East Antarctic Ice Sheet (EAIS) that underwent large size variations at orbital time scales. The dynamic behavior of the EAIS implies that a significant proportion of the variability seen in oxygen isotope records of the late Miocene reflects Antarctic ice-volume changes.

Seismic and Side-Scan Sonar Investigations of Recent Sedimentation in an Ice-Proximal Glacimarine Setting, Kongsfjorden, North-West Spitsbergen.

The sediments and rates of deposition adjacent to the margins of tidewater glaciers have been described from a range of glacimarine settings, including south-east Alaska, Baffin Island and Svalbard [e.g. Elverhoi et al., 1980; Syvitski et al.,1987; Powell and Molnia, 1989]. One purpose of these studies is to present evidence from modern glacier-influenced environments, where the glaciological setting is either observable or relatively readily inferred [Dowdeswell and Scourse, 1990]. This information can then be utilised in the interpretation of Quaternary and older glacimarine sediments, where the depositional setting must be inferred on the basis of seismic stratigraphy and sedimentary facies.

Origin of shallow submarine mass movements and their glide planes—Sedimentological and geotechnical analyses from the continental slope off northern Norway

Submarine landslides are often characterized by a basal surface of rupture parallel to the stratigraphy, in which downslope movement is initiated. However, little is known about the sedimentology and physical properties of the sediments within these surfaces. In this study, we present a multiproxy analysis of the sediments collected from a giant piston core penetrating a shallow submarine mass transport deposit, in combination with high-resolution seismoacoustic data to identify and characterize the basal glide plane and the weaker sediments in which movement was initiated. The initial phase of instability consists of a single fracture that formed due to the downslope movement of a mostly intact slab of sediments. The 16 m long core, comprising mostly undisturbed massive and laminated ice-rafted debris-rich clay penetrated this slab. The base of the slab is characterized by a high-amplitude semicontinuous reflection visible on the subbottom profiler data at about 12.5 m depth, interpreted to originate from the glide plane on top of a plumite deposit. This plumite has dilative behavior with pore pressure decrease with increasing shear strain and high undrained shear strength. Movement probably started within contouritic sediments immediately above the glide plane, characterized by higher sensitivities and higher water contents. The occurrence of the mass movements documented in this study are likely affected by the presence of a submarine landslide complex directly downslope. The slide scar of this landslide complex promoted retrogressive movement farther upslope and progressive spreading of strain softening along the slide base and in the slide mass. Numerical models (infinite slope, BING, and retrogressive slope models) illustrate that the present-day continental slope is essentially stable and allow reconstruction of the failure processes when initiated by an external trigger.

Multidisciplinary investigation of a shallow near-shore landslide, Finneidfjord, Norway

The 1996 landslide near Finneidfjord, Norway, involved the displacement of c. 1 x 106 m3 of sediment. Failure initiated offshore and developed in a retrogressive manner, back-stepping 100 – 150 m inland, and removing a 250 m long section of the main North-South highway. The landslide caused the loss of four human lives, and may have been triggered by human activity (e.g., blasting for road works and/or placement of fill along the shore). Acquisition of an extensive and multi-disciplinary data set, including high-resolution swath bathymetry, 2D/3D seismic data, multiple short (up to 6 m) and two long (12 m and 14 m, respectively) sediment cores, and in situ Free-Fall Piezocone Penetrometer (FF-CPTU) profiles complemented with geotechnical laboratory data, has afforded detailed analysis of both the landslide morphology and stratigraphic controls. Using regional 2D parametric sub-bottom profiler (TOPAS) profiles and a targeted decimetre-resolution 3D Chirp seismic volume (950 m x 140 m), we focus on post-failure material transport/deposition, correlating the failure plane against one of several regionally extensive packets of high–amplitude, composite reflections. In seismic reflection data, the slide plane lies within a distinct, thin (< 0.5 m) stratigraphic bed of lower acoustic impedance than the background sedimentation (indicated by high amplitude reverse-polarity top reflection), which is extensively deformed or completely scoured by motion of the overlying material. Within the body of the landslide, two different flow facies are identified. Inversion of these broadband (1.5 – 13.0 kHz) seismic data has allowed the calculation of remote physical properties (using acoustic quality factor, Q), affording a depth and spatial assessment of the relationship between morphology and grain size. These remote physical properties have been correlated against high-resolution geotechnical data from core logs and FF-CPTU profiles, identifying the slide plane as a weak, laminated, clay-rich bed. This combined geophysical/geotechnical assessment of the landslide morphology and internal architecture supports previous work indicating a complex, multi-stage failure. These combined data illustrate how seafloor stability is strongly influenced by shallow subsurface structure, with the geotechnical properties and lateral continuity of stratified beds acting as a primary control on slide plane depth and failure probability.

Shallow Landslides and Their Dynamics in Coastal and Deepwater Environments, Norway

In this manuscript, we present the first results of integrated slope stability studies to investigate smaller-scale mass movement processes in different physiographic settings of Norway. These include coastal areas (Sorfjord, Finneidfjord), and pristine open ocean settings in intermediate (Vesteralen) and deep waters (Lofoten) on the Norwegian margin. Triggers, pre-conditioning factors and sedimentary processes associated with these landslides are currently not well constrained.

Possible consequences of glacially induced groundwater flow

Abstract Groundwater flow induced by a glacier representative of the last and previous glaciations was modelled through a simplified geological section in the Barents Sea. A pervasive Jurassic sandstone was assumed to be a continuous aquifer sandwiched between older and younger aquicludes (claystones). Results show that the aquifer may have a capacity to drain up to several orders of magnitude more water than that which is produced locally at its subglacial outcrop, whereas the aquicludes can only drain an order of magnitude less water. This implies that drainage of water from beneath the glacier and subglacial springs may have an influence on glacier dynamics through control of the compaction and thereby the shear strength of subglacial tills. The numerical analyses show that the resultant groundwater flow through the aquifer may cause a tilting of an oil/water contact of up to about 1:10.

Finneidfjord: a Field Laboratory for Integrated Submarine Slope Stability Assessments and Characterization of Landslide-Prone Sediments: A Review

Sorfjord outside the village of Finneidfjord has a history of landsliding throughout the Holocene. The 1996 landslide – the focus of this study – has many characteristics typical of submarine landslides (well-developed slip plane, outrunner blocks, peripheral thrusting and lateral spreading). Due to its sheltered and accessible location, Finneidfjord has become a natural laboratory for testing high-resolution and multidisciplinary techniques to improve our understanding of landslide development. This study integrates multiple sediment cores, swath-bathymetry surveys, single- and multi-channel 2D seismic data (Topas, boomer, sparker, airgun), very-high-resolution 3D chirp seismics, ocean-bottom seismometer as well as free fall and traditional cone penetration testing (CPTU). The cores have been subjected to both geological and geotechnical laboratory analyses. Of particular interest is the correlation of the regional slip plane as a high-amplitude package of reflections in the geophysical data with the results of the sediment and in situ measurements. Comparison of 3D traces with synthetic seismograms based on multi-sensor core logs show that the most prominent slip plane lies within a thin clay unit sandwiching a sand seam. The slip plane is difficult to identify from CPTU data alone. The top part of this composite unit has in places been eroded under the 1996 mass-transport deposit (MTD). This composite unit’s formation is associated with turbidite deposits from terrestrial quick clay landslides and possibly river floods in the catchment of the fjord. While the MTD is extensively deformed, different flow facies are identified within the landslide body revealing a complex, multi-phase failure. The seismic data were also used to infer physical properties (mean grain size, gas saturation from P-wave attenuation). Interestingly, shallow gas adjacent to the landslide appears not to have played a role in the landslide development. Fjordbed stability is strongly influenced by shallow subsurface structure, with geotechnical properties and lateral continuity of stratified beds acting as primary controls on slide plane depth and failure mechanisms. This study can well form a template for near-shore areas prone to landsliding. Currently, a long-term pore pressure monitoring programme is in progress, after the installation of several piezometers close to the depths of the slip plane close to the shoreline in September 2012.